In the large output of a battery system, a high voltage is obtained by connecting a lot of battery cells in series. In this battery system, the series connected battery cells are charged by the same charging current, and discharged with the same discharging current. Therefore, when all of the battery cells have exactly the same battery property, unbalances in the battery voltages or the remaining capacity do not occur. However, the battery cells having exactly the same battery property, cannot be actually manufactured. When charging and discharging are repeated, the unbalance of the battery cells occurs in the unbalances in the battery voltages or the remaining capacity. Further, the unbalance in the battery voltages causes over charge or over discharge in a specific battery cell. In order to prevent over charge or over discharge of the battery cells, a battery system which detects the voltage of each of the battery cells and controls current of charge and discharge, has been developed. (refer to patent literature 1)